1. Field of the Invention
The present invention relates to a hard film, which is formed on a surface of a cutting tool such as a tip, a drill, and an end mill, and a surface of a plastic working tool such as a forging die and a punch for improving wear resistance of the tools, and relates to a method useful for manufacturing such a hard film.
2. Description of Related Art
Usually, coating of a hard film of TiN, TiCN, TiAlN or the like has been performed for the purpose of improving wear resistance of a cutting tool using sintered hard alloy, cermet, or high speed tool steel as a base material. In particular, since a composite nitride of Ti and Al (hereinafter, abbreviated as “TiAlN”) exhibits excellent wear resistance as disclosed in Japanese Patent No. 2644710, a film of the composite nitride is increasingly used for a cutting tool for cutting a very hard material (work material) such as a high speed cutting material or hardened steel in place of a film including nitride (TiN) or carbonitride (TiCN) of Ti.
However, a film further improved in wear resistance is now required with recent increase in hardness of work material or increase in cutting speed.
The hard film is further required to have oxidation resistance under high temperature. In the TiAlN film as above, oxidation resistance is comparatively high, and oxidation starts near 800 to 900° C., however, there is a difficulty that deterioration of the film tends to progress under a more severe environment. Therefore, a hard film is proposed, in which the TiAlN film is added with Cr, thereby the concentration of Al is increased while keeping a cubic crystal structure with high hardness, and consequently oxidation resistance is further improved (e.g., JP-A-2003-71610). Moreover, a hard film is proposed, in which oxidation resistance is further improved by adding Si or B into a TiCrAlN film (e.g., JP-A-2003-71611), or a hard film is proposed, in which oxidation resistance is further improved by adding Nb, Si or B into a CrAlN film (e.g., WO2006-005217) is proposed.
However, the hard films proposed so far cannot be regarded to be excellent in wear resistance and oxidation resistance, and actually, further improvement in properties is desired.